It is usually a good idea to double-space.

Double-spacing means writing on one line, then leaving the next blank.

If you have big, round handwriting, double-spacing gives you room to add to your writing in the revision phase. If your handwriting is tiny and cramped, double-spacing encourages you to write a bit bigger.

It also gives your teacher room to give you some feedback on your writing skills so you can improve. You are at school to learn skills and get better at them--and if you don't get feedback, you can't do that as easily.

If you forget to double space, or stop double-spacing because you turned the page and forgot, don't panic! Simply continue double-spacing as soon as you realize your mistake. Your teacher PROBABLY won't be TOO grumpy about it as long as you are making an effort :).